1. Field of the Invention
The present invention relates in general to coupled line devices such as may be used in constructing quadrature hybrids and couplers including directional couplers. More particularly, the present invention relates to an improved microwave coupler construction having improved temperature thermal expansion characteristics.
2. Background Discussion
A highly effective form of microwave coupling device applicable to a range of frequencies, centered around a predetermined frequency, involves the use of a pair of wire like inner conductors with at least one of the conductors containing insulation. These two inner conductors are placed in contact with each other along their lengths and are usually slightly twisted around one another to maintain their relative positioning. About this pair of inner conductors is placed a pliable insulating sleeve composed usually of a Tetrafluroethylene (TFE) based material such as Teflon.RTM.. This sleeve and the conductors therein are housed in a shielding outer conductor composed generally of a copper-based alloy or aluminum. Such a microwave conductor is disclosed in U.S. Pat. Nos. 3,358,248 and 4,547,753.
A significant problem with such a microwave coupler is that it is subjected to a fairly wide range of operating temperatures (as much as -55.degree.-+85.degree. C.) during use. Higher internal temperatures may be attained with high energy couplings or even during soldering of the coupler onto a printed circuit board. As such, the parts of the coupler tend to expand and contract at various times during the operation. Since the coupler is usually elongated and open at either end, expansion is particularly pronounced in an axial elongation direction. The outer conductor, is generally composed of a copper-based alloy or aluminum. Copper alloy has a coefficient of linear thermal expansion in the operating temperature range of approximately 1.6.times.10.sup.-5 /.degree.C. Aluminum has a coefficient of approximately 2.4.times.10.sup.-5 /.degree.C. In contrast, standard Teflon.RTM. has a coefficient of linear thermal expansion of approximately 9.times.10.sup.-5 /.degree.C. Thus, any expansion of the outer conductor may be amplified by more than five times in the Teflon.RTM.. This differential expansion can cause the insulating sleeve to significantly bulge out of either end of the outer conductor tube resulting in mechanical strain and islocation of the inner conductor wires.
A typical microwave coupler 100 mounted on a circuit board 101 is shown in FIG. 4. The outer conductor leads 102, 104, 106, 108 extend out opposing uncapped ends 110, 112 of the coupler 100. The outer conductors 102, 104, 106, 108 are encased in an insulating sleeve 114. The metallic outer conductor 116 snugly fits over the sleeve 114. The leads 102, 104, 106 and 108 are each mounted to a respective laminated metallic strip 118, 119, 121, 123 on the circuit board 101 generally by means of a solder joint 120. Note that in this four part coupler example, the shield, terminals 106, 108, input 122 and output are each mounted to a common ground. When the coupler operates to transfer RF energy from an input 122 to an output 124, a large amount of heat is generated in the conductor 100 that results in the linear expansion (arrows 126) of the outer conductor 116 and the significantly greater expansion (arrow 128) in the sleeve 114. The effect of expansion is herein illustrated for one end 110 of the coupler 100, but one should assume that the effect is experienced equally by the other end 112. This expansion places pressure (arrows 130) upon the conductor leads (for example 102, 104). Since the leads 102, 104, 106, 108 should remain relatively short to attain good coupler performance, the leads are somewhat taut. Thus, the sleeve pressure 130 creates tension (arrows 132) in the leads 102, 104 consequently placing strain upon the solder joints 120. As noted above, the cyclic loading of these joints may eventually result in loosening or total disconnection of leads from the circuit board strip 118 or 119. Even if the leads experience low tension, the preferred method of mounting allows the sleeve pressure 130 to impart a cyclically acting moment (curved arrows 134) about the solder joints 120. The leads are, in effect, a lever with a fulcrum at the joint 120 and a distal end at the sleeve 114. Eventually, the moments also result in fatigue and joint breakage.